Automatic focus (hereinafter “autofocus” or “AF”) is a feature of some optical systems that allows them to obtain and in some systems to also continuously maintain correct focus on a subject, instead of requiring the operator to adjust focus manually. An autofocus scan often uses a computer to run a miniature motor that moves a lens to various positions to focus the lens for you. Focusing is the moving of the lens in and out until the sharpest possible image of the subject is projected onto the film or other image sensor. Depending on the distance of the subject from the camera, the lens has to be a certain distance from the film to form a clear image.
Autofocus systems rely on one or more sensors to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern SLR cameras use through-the-lens optical AF sensors, which also perform as light meters. Active AF systems measure distance to the subject independently of the optical system, and subsequently adjust the optical system for correct focus. These systems use what may be characterized as a range detection system to measure distance to the subject. Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. These systems (e.g., contrast detection) generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject.
Contrast measurement is achieved by measuring contrast within a sensor field, through the lens. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximum contrast is detected. In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light.